Observation of dense fields or extended objects

When the science target is an extended object or is located in a crowded field then the contribution of the sky background has to be sampled away from the science object. This results in a loss of efficiency for the observations, which can amount up to 50% of the time if the sky has to be sampled as frequently as the object. In what follows we will show an example for such observing strategy (i.e. sky measurements done off target), which is suitable in case of extended objects or crowded fields. In particular, we will use an observing template called FixedSkyOffset that allows you to move back and forward between the target and a sky position. However, measuring the sky background off-target can be performed by using other different templates (i.e. GenericOffset, AutoJitterOffset), hence the reader should also refer to the HAWK-I template manual as well as to the HAWK-I User manual to decide what template is most suitable.

In case your science target does not require to perform large offsets to monitor the sky background then click here to return to the HAWK-I main tutorial page.

0: Goal of the Run

In this tutorial we will prepare an OB that perform imaging observation of a dense stellar cluster in the JKs filters. Your goal here is to obtain precise photometry for the stars in the cluster which has a spatial extent of ~5arcmin and whose approximate center is at RA(2000) = 03 15 00.31, DEC(2000) = -07 24 25.4. For convenience hereafter we will call the cluster ESO-1 (Note: This is a fictional stellar cluster).

The sample OB will illustrate the use of a variety of features of P2PP (version 3) and illustrate the kind of decisions to be taken at the time of preparing in advance of an observing run, as well as some aspects that are specific to the preparation of OBs for HAWK-I.

1: Getting started

The Phase 2 process begins when you receive an email from the ESO Observing Programs Office (OPO) telling you that the allocation of time for the coming observing period has finalized and that you can view the results by logging into the UserPortal and clicking on "Check the webletters." Note that the username and password that you need to use for the User Portal are the same as those you will use to prepare your OBs.

You follow the instructions given by ESO and find that time was allocated to your run with HAWK-I. Therefore, you decide to start preparing your Phase 2 material. First, you collect all the necessary documentation:

2: Your First OB

2.1: Define an OB with P2PP

For the sake of this tutorial, we will hereafter use the following P2PP information:

P2PP ID: 52052

password: tutorial

This is a special account that ESO has set up so that users who do not have their own P2PP login data can still use P2PP and prepare example OBs. You cannot use this account to prepare actual OBs intended to be executed.

After starting P2PP and logging in using the tutorial account, the P2PP main GUI will appear as follows:

Runs for a number of instruments appear in the Name area, since the same tutorial account is used for all of them. Similarly, if you log in with your own P2PP ID, then select Download/Refresh Obs.runs from the File pull-down menu and you will get the list of all the runs in which you are PI.

Select the folder corresponding to the HAWK-I Tutorial run, 60.A-9253(L). In this tutorial we assume that time was allocated in Service Mode. This is indicated by the SM letters that appear next to the RunID of the HAWK-I run.

You can now start defining your OB.

Click on the OB blue icon on the upper left side of the P2PP main GUI. This creates an entry under the Observing Runs area. The red dot next to the OB name means that the OB fails to pass some fundamental verification criteria, as may be expected from the fact that no template has been attached to the OB yet.

Now the next step should be naming the OB, however because in some special cases a particular prefix must be given to an observing OB, you check first on the Service Mode Guidelines webpage if a special naming convention must be given to your HAWK-I imaging OB. Consulting that page you confirm that your observations do not need any particular OB naming prefix, because it is neither a target-of-opportunity object, nor moving asteroid, nor anything else that would require a particular OB naming prefix. Hence, you decide to name your OB: ESO 1-JKs. Select the OB, press Enter and you type ESO 1-JKs in the No Name field.

The P2PP main GUI should then appear as follows:

Double click on the OB name to have access to the main OB window where you will define the contents of your OB. At this point the OB main GUI should appear as follows:

2.1.1: Filling in the Basic Information

OD Name

It may be useful in many cases to have an easy way of identifying an OD (Observing Description), like when having observations of a number of targets performed with an identical instrument configuration and observation template parameters. The OD Name field in the View OB window allows you to define names for the ODs. The OD name appears in turn in the Obs/Calib Blocks area of the P2PP main GUI, thus allowing the identification at a glance of all OB sharing ODs with the same name. In this tutorial the observation description will define the imaging observations with the J and Ks filters. Thus, we enter the name JKimaging in the OD Name field.

User Comments

The User Comments field can be used for any information you wish: to keep further track of the characteristics of the OB, to alert the staff on Paranal to special requirements, etc. Here, we do not have any special information or requirements and leave this field blank.

Instrument Comments

The HAWK-I specific field refers to the Magnitude of the brightest object in the field hence it must be used to insert the J, H, and K magnitude of the brightest star present in the 7.5' x 7.5' HAWK-I field. In order to retrieve this information you query the 2MASS catalogue. When querying the catalogue you do not provide a search box size of 7.5' x 7.5' only, but an even larger size of 8' x 8' to account for the fact that you will define (see below) also a jitter box width of 60" in your science template. The query result tells you that the brightest object in the whole field has the magnitudes: J=11.2, H=11.0, K=10.7mag. You verify on the HAWK-I service mode specific webpage that this is not too bright for HAWK-I observations. Note that imaging observations leading to strong (larger than a factor of 5) detector saturation are not schedulded in service mode. You continue with typing J=11.2 H=11.0 K=10.7 into the Instrument Comments field.

2.1.2: Defining the acquisition template

The first template that must be part of any science OB is the acquisition template, so let us define it next. In the Template Type list, make sure that the acquisition entry is selected. This will list all the acquisition templates available for HAWK-I in the Template list below to it.

After reading the description of the templates in the HAWK-I template Manual, you have noticed that the HAWKI_img_acq_Preset template is the acquisition template most suited for your purpose, because a very precise target centering is not neccessary, nor you are performing RRM observations. Therefore you click on the template named HAWKI_img_acq_Preset from the list of possible templates in the Template list. Then, you click on the Add button next to it. The window should now look like this:

Now, you need to decide on the acquisition parameters, and if necessary, modify the default values given in the acquisition template.

The first fields to fill out in the acquisition template are the DIT (detector integration time for a single read) and NDIT (number of DITs to co-average) settings. Your decision for the values of DIT and NDIT depends on the brightness of the target. Since your stellar cluster contains bright stars and is easily identifiable in a short exposure of 10 seconds at J-band, you enter:

DIT (secs): 10

NDIT: 1

The next three parameters of the acquisition template are related to the orientation and positioning of the field before the first imaging exposure is taken. In our tutorial example we assume that a very interesting object is present very close to the cluster center. This object would fall into the gap between the HAWK-I detectors during the acquisition. In order to prevent this the parameters Alpha Offset for the target and Delta Offset for the target are given a value, so that the telescope executes an offset before starting to take the science images. In our example, we choose to offset 15 arcsec in RA and 15 arcsec in DEC. The rotation of the field, which is defined by the parameter Position Angle on Sky (deg), is not important for us as the cluster is quite symmetric, and you decide to keep the default value of 0 deg. So, you enter:

Alpha Offset for the target: 15

Delta Offset for the target: 15

Position Angle on Sky (deg): 0 (Default value)

Next, you have to decide about the telescope guide star. It may happen that one wants to observe a region in a large dark cloud where no optically bright star is present that can be used for the telescope active optics guiding. In such a case the parameter Telescope Guide star selection must be set to NONE. However, in the large majority of cases, and also in our tutorial example, thre is no problem for the telescope operator to automatically find a telescope guide star from the catalogue. Therefore, in most cases and so for our example here, this field can be left at its default value:

Telescope Guide Star Selection: CATALOGUE (Default)

Only in case the Telescope Guide star selection is set to SETUPFILE the values for "RA/DEC of telescope guide star" (the next acquisition template keywords) are used.

Finally, you have to enter the filter that should be used for acquisition. It is recommendable to chose the same filter as you will use in your first science template to decrease the overheads, and because you decide that this will be J-band, you choose from the filter drop-down menu:

2.1.4: Setting the Constraint Set

As stated in Section 1, we assume for the purpose of this tutorial that the program has been allocated time in Service Mode. You thus need to specify a set of constraints, which indicate under which conditions your OB can be executed. You can do this by clicking on the Constraint Set icon which will give you access to the view window that looks like this:

Now you should fill all the entries specific to HAWK-I observations (i.e. only the first 6 entries)

Name: give a descriptive name to the constraint set about to be defined. Since you have decided that this constraint set will be applied to all the J and Ks imaging observations, you type JK_constraintsin the Name field.

Sky transparency: since you wish to be able to determine accurate fluxes from your images, you request Photometric conditions in the Sky Transparency entry.

Seeing: since you need moderately good quality in your images, you specify 0.8 as the value of the Seeing field.

Airmass: set the airmass to 1.6 to ensure that your observations are not carried out at too low elevation.

Lunar illumination and Moon Angular Distance: since you are doing broad band observations in the near-infrared, the lunar illumination has very little influence. You can thus leave the default values of 1.0 and 30 degrees for the Lunar Illumination and Moon Angular Distance fields.

Twilight (min): Because HAWK-I is an infrared imager, observations of bright objects may be carried out in twilight. From P91 onwards, there is a new constraint in P2PP3 called twilight constraint. This constraint can be used to define the earliest time with respect to the end of the astronomical twilight when the execution of the OB can be started. While the relation between the time difference from the evening twilight end and sun elevation varies during the year, for Paranal due to its low latitude this difference is small. Therefore the constraint is given in minutes as a difference in time with respect to the end of astronomical twilight (i.e. the time when the solar elevation is -18 degrees). The default value of twilight constraint is 0. A negative number means that it is allowed to start the observation before the end of the astronomical twilight. The twilight constraint can take values between -45 and 0 minutes. In this specific case, your observation can be done during twilight time, as soon as the Telescope Operator manages to acquire a suitable guide star (typically about 30 min before the end of twilight) hence you specify -30 as the value for the Twilight constraint.

NOTE: In your Phase I proposal you already specified some of these constraints (lunar phase, seeing, and transparency). At Phase II, you can relax your constraints to improve the chances of execution of your programme, but you cannot specify more stringent constraints.

2.1.5: Setting the time intervals

We will assume now that your HAWK-I observations are part of a larger multi-wavelength project and that the HAWK-I observations should be carried out simultaneously with some satellite observations that are performed in October: between 02-10-2012 and 15-10-2012 and in late December: between 15-12-2012 ans 23-12-2012. You can specify this in the Time Intervals view window, which is accessible by clicking the Time Intervals icon. See below

To provide the first time window for the observation click the New TI button. A pop up window will appear giving you the option to select the date and time of the start and end of the observation.

Select 2012-10-02 and 2012-10-15 for Start and End date, respectively and then press ok

The Time Intervals view window should look like this:

Now you can repeat the exsercise to add the second time critical window and providing as Start and End date for the observations: 2012-12-15 and 2012-12-23, respectively.

When you have done filling with the time interval constraints, the view window should appear as follows:

2.1.6: Defining the Obs.Description / Science templates

Once the view window Target, Constraint Set and Time Intervals are completed, the science template(s) can be inserted. To do so go back to the Obs. Description view window. In this example OB, we will insert two science templates (one after the other), as we wish to define an observing sequence of exposures with the J and Ks filters.

On Template Type, select now science. The existing HAWK-I science templates will appear.

Now you have to make a choice between the different HAWK-I imaging templates. Such choice is driven by the scientific requirements of your programme and the characteristics of your target. Since the stellar cluster we are considering in this tutorial example is large in extent and very rich, it will be impossible to obtain a good estimate of the sky background from the cluster field itself. A good strategy is to obtain separate sky offset fields far away from the cluster. You consult the Template Reference manual and you select the template HAWKI_img_obs_FixedSkyOffset from the template list. Click on the Add button to attach the template to the grid below next to the acquisition template selected and filled previously.

The OB view window should look like this now:

In order to fill out all parameters of this observing template you consider the following: using the information provided by a near-infrared survey (e.g. 2MASS) you find out that a sky field 510 arcsec East and 340 arcsec South of your cluster has a relatively low stellar density and does not contain any bright stars. Hence, this field is perfectly suited as a sky field. Further, you want to apply a random jitter pattern within a 60 arcsec box on your object and your sky fields. You also want the brightest source in your field not to saturate heavily (no more than a factor 5 above saturation level), but still want to obtain a sufficient signal to detect fainter sources. After consultation of the manual and the Exposure Time Calculator you find that you will approximately need a total on-source time of 480s per filter, and that the recommended DITs for J are 10s. In addition, you recognize the rule that the minimum time spent at a jitter position (=DIT * NDIT) is 60s. You decide to set DIT to 10s and NDIT to 12 for the J band. The SATLEVEL keyword is set per default to 25,000 and you leave it like this.The final strategic consideration concerns the sky fields: you see that more than 3 AB cycles should be obtained to be able to construct a median sky. So, you decide to define 4 AB cycles, this results in 4 jitter positions on the sky and on the object, respectively. For the sky fields you wish to spend only half of the observing time, and hence you set NDIT to 6 for the sky. Furthermore, you decide to start the jitter in each filter at the reference position given by the acquisition, rather than at the last position observed in the previous template. The first HAWKI_img_obs_FixedSkyOffset template (the observation in J) thus has the following parameters:

DIT (secs): 10

SATLEVEL: 25000 (Default)

Number of exposures per offset: 1

Return to Origin ? (T/F): checked (i.e., True) (the telescope will thus return to the initial position after the execution of the template)

Observation Category: SCIENCE (Default)

Jitter Box Width (arcsec): 60

Number of AB or BA cycles: 4

NDIT on OBJECT positions: 12

NDIT on SKY positions: 6

Type of first observation: O (Default)

RA offset to sky (arcsec): 510

DEC offset to sky (arcsec): -340

Filter Name: J

You can already check the execution time of this OB by clicking the Recalculate button on the right side of the Execution Time field.

... and the template section in your OB window should look like this now:

For Ks band, you rather want to set the DIT to 5s, NDIT to 24 for the object positions and NDIT to 12 in case of sky positions.

For the observations in Ks, you could select again the same template, Add it, and fill the parameters in the same way as done for the template in J. However, since the parameters of this template will be very similar to those of the one just defined, you can speed up the preparation by clicking on any entry of the template for the J-band observation, then clicking on the Duplicate button on the right. In this way, you will have produced an identical copy of the first science template in which you should now only edit the parameters that change from template to template:

DIT (secs): 10 must be changed to 5

Return to Origin ? (T/F): not checked (i.e. False)

NDIT on OBJECT positions: 12 must be changed to 24

NDIT on SKY positions: 6 must be changed to 12

Filter Name: J must be changed to Ks

If you do not check the Return to Origin ? (T/F): box, because offsetting back to the original position is not needed, then neglecting this offset will save you OB execution time.

Now you are almost done. Check the final execution time of the OB as done previously (i.e. click the Recalculate button) which will give you the time needed to execute your observation. If you have followed all the indications given so far, the View OB window should look like this now.

2.1.7: Verifying the OB and setting its priority

You can now close the OB window and go back to the P2PP main GUI. In there, under the Obs/Calibs Blocks subfolder you should see an entry with the following contents:

Name: ESO 1-JKs

Status: (P)artiallyDefined

Target: ESO-1

OD: JKimaging

CS: JK_constraints

Acquisition: HAWKI_img_acq_Preset

FindingCharts: (0)

EphemerisFile:

You can reshape the columns as indicated in the P2PP User Manual to view the full contents of each entry. See below

Now to verify that your OB is free from error you can select Verify option from the Reports pulldown menu in the upper bar of the P2PP main GUI. This will produce a log message report listing possible warnings and/or errors that you should take care of. If you have followed step by step this tutorial then the log message should not report neither errors nor warnings.

By selecting the Execution Time option under the Reports pulldown menu you can check both the Execution and Exposure time of your OB, where the latter refers to the time free from overheads

Under the Schedule subfolder you should then see an entry with the following contents:

Name: ESO 1-JKs

Priority: 1

Contrib. to group:

Abs. Time Intervals: 2

Earliest After Prev.:

Latest After Prev.

See below:

In case your run contains more than one OB (very likely), you can select a priority for this OB which is currently set to 1 as default. The priority is a number ranging from 1 to 10, where 1 corresponds to the highest priority. You can assign to your OB a priority by clicking on the number diplayed for your OB in the P2PP main GUI under the Priority column.

3:Attaching Finding Charts

The next thing to do is to attach the respective Finding Chart(s) to the OB. The Finding Charts must be prepared as jpeg or jpg files and must fulfill all general and HAWK-I specific requirements for finding charts. You can use any tool of your choice to create the Finding Charts in jpeg, jpg format. P2PP, however, does not contain such option.

Let's assume you have prepared a jpg-Finding Chart for this tutorial run [remember: run ID 60.A-9253(L)], which you called 60.A-9253L.ESO-1.jpg, and which is saved in a sub-directory of your home directory.

Now, in the P2PP main GUI click on the OB that you want to associate with this finding chart, then select Finding Charts from the top menu bar, which opens a drop-down menu as you see here below:

From the drop-down menu select Attach Finding Charts, which will open up a new window that allows you to enter path and filename of the Finding Chart you wish to attach to the selected OB. In our example you choose 60.A-9253L.ESO-1.jpg

Finally click on the Attach Finding Charts button (you could select more than one Finding Chart). The pop-up window will close and in the P2PP main GUI, Obs/Calib Blocks under the Finding Chart column, you will see the entry

Finding Charts: (1) 60.A-9253L.ESO-1.jpg

If you are interested in a more comprehensive explanation on how to create and attach or detach finding charts, you should have a look at this page.

4: Finishing the preparation and submitting the OBs

With the completion of the OB, we consider the example developed in this tutorial to be finished. The P2PP main GUI displays the OB that we have prepared:

We will now submit the OB to the ESO Database: select the OB, go to the File menu in the P2PP main GUI, and select the Check-in option. A dialog box will appear asking for confirmation and, if you click on OK, it will be saved in the ESO Database.

Our tutorial with this example of creating and submitting the HAWK-I OB for JKs imaging using a fixed sky offset ends here. For the preparation of the Phase II material for a whole run, more OBs may have to be created. Furthermore, the complete Phase 2 package lincludes also the README file. As of P75, the README file is submitted along with the OBs by using the P2PP tool. A tutorial for the README file is available here . When all the OBs and the README file for a given run are submitted, the Phase II submission is finalized by pressing the Whistle icon.

As a courtesy to the next user who follows this tutorial, we would like to ask you to finish these exercises by removing the OBs form the ESO Database. The P2PP User Manual gives you detailed directions on how to do this. In short,

Select Check-out from the File menu in P2PP.

In the Database Browser window that opens, type 60.A-9253(L) in the Prog ID selection criterion

Click on the Query button on the lower left.

Select all the OBs that appear in the display area after the query. Normally there should be your submitted OB only, but if another user has submitted other OBs from this same account without removing them afterward you will see them as well.

Under the File menu in the OB window, select Check-out.

In this way the OBs will be removed from the ESO Database and will be left in your Local Cache only. From there you can delete them if you like by selecting them and choosing the red X icon on the upper bar of the P2PP main GUI.